(a) Field of the Invention
The present invention relates to vacuum fluorescent displays (VFDs), and more particularly, to improvements allowing a higher efficiency of electron function to enhance the display quality.
(b) Description of the Related Art
In recent years, VFDs, utilizing phosphor display elements to form a viewed alphanumeric or graphic image, have come into wide use as displays in electronic and electrical appliances.
A typical vacuum fluorescent display device comprises a transparent evacuated envelope containing a plurality of anodes arranged in a pattern of desired light emission, each anode being coated with a fluorescent layer for emitting light when excited, a heated filament serving as a source of electrons, and control grids located between the filament and the anodes for determining which anodes can be excited by the electrons. When the anodes and the control grids are at a high voltage and the filament is at a lower voltage the electrons can excite the phosphor layer on the anodes to cause light emission from the anodes.
Referring to FIG. 5, a conventional VFD will be described. An evacuated envelope is sealed with a face glass 2, a base substrate 4 and side glasses 6. The base substrate 4 comprises a wiring layer (not shown) covered with an insulating layer 8. A conducting layer 12 (anode) is formed on the insulating layer 8 and is provided with a positive potential through a conducting wire 10. A phosphor layer 14 is deposited on the conducting layer 12.
A plurality of filamentary cathodes 18 is located in the envelope spaced from the anode 12 and is heated to thermionically emit the electrons. Control grids 16 are located between the anode 12 and the cathode 18 to accelerate the emitted electrons.
In the VFD shown in FIG. 5, or other similar triode vacuum tubes, the filament is heated, such as by an AC current, to a temperature at which it will emit electrons. The control grids, biased at a positive potential, accelerate electrons emitted from the filament toward the anode, which is also biased higher than the filament. On the anode, the phosphor layer emits light in response to the bombardment by electrons emitted from the filament and accelerated by the control grid to the anode.
Since only those electrons that pass through a positive control grid can reach the anode, some electrons that do not pass are absorbed by the control grid. That is, since only some of the emitted electrons impinge on the phosphor layer for lighting, the efficiency of electron function is not optimal, and this results in a degradation of the brightness of the display.
Further, when the control grids are hit by the electrons, they may be heated by additional currents and suffer thermal deformation. Even if the spaces between grids are partially shifted, stains or spots on the display pattern will result.
Because the control grids as well as the anode are formed on the base substrate, the design of VFDs may be restricted by space availability. The components comprising grids and anodes are formed on the base substrate, and it subsequently takes a long time to manufacture a VFD.
In view of the prior art described above, it is an object of the present invention to provide a vacuum fluorescent display capable of reducing the absorption of electrons by a control grid to enhance the efficiency of emitted electrons.
It is another object of the present invention to provide an improved vacuum fluorescent display having enhanced brightness and higher display quality.
To achieve these objects, as embodied and broadly described herein, the invention comprises
an evacuated envelope surrounded by a pair of substrates and side glasses;
a plurality of filamentary cathodes for emitting electrons when a negative potential is applied; and
a display unit, provided on one of the substrates in the evacuated envelope, having a positive potential applied thereto, and displaying a predetermined image in response to the electrons emitted from the plurality of filamentary cathodes;
an electron control unit for generating a repulsive electric field to allow acceleration of the electrons emitted from the plurality of filamentary cathodes in the direction of the display unit.
A negative potential is applied to the electron control unit. The electron control unit may be a plurality of grids that are shaped as a mesh, or it may be a layer of a transparent electrically conductive material such as tin doped indium oxide (ITO) deposited on the other substrate.
According to another aspect of the present invention, a VFD further comprises a control electrode, located around the electron emissive means, for controlling the trajectories of the electrons emitted from the electron emissive means.
Both the foregoing general description and the following Detailed Description are exemplary and are intended to provide further explanation of the invention as claimed.